1. The Field of the Invention
The present invention relates to data storage on rotating magnetic storage devices. More specifically, the present invention relates to methods used in calibrating and selectively positioning a bidirectional recording head in a magnetic storage device to enable greater positional accuracy during device operation.
2. The Related Technology
During recent years, there has been a steady improvement in the volume of data that can be stored on magnetic storage media, such as hard disk drives used in computers. Today, a single 3.5 inch magnetic storage disk can store twenty gigabytes or more of data. At the same time, storage capacity per unit cost has fallen dramatically, which has enabled individual users and enterprises to radically change the way in which data is recorded and stored. Indeed, the ability to store large volumes of data inexpensively has been a driving factor in the information technology revolution during recent decades.
Conventional storage media include solid-state devices, drive arrays (RAID), single rotating magnetic disk drives, and removable optical media. FIG. 1 is a graph that illustrates tradeoffs between performance and cost associated with typical storage media used in combination with computers. As shown, removable optical storage devices, such as optical read-only or read-write disks, generally provide the least expensive alternative for storing large amounts of data. However, single rotating magnetic devices, such as hard disk drives used in large numbers of personal computers, provide mass storage that is almost as cost effective as removable optical devices, but with better performance. In this context, the term “performance” relates primarily to the reliability and access times associated with the various storage media. As shown in FIG. 1, however, the performance of single rotating magnetic storage devices is increasing less rapidly than the performance of RAID and solid-state devices.
Although magnetic storage devices are widely used and have become significantly less expensive during recent years, a number of technological hurdles have been encountered, which threaten to reduce the rate at which future improvements in cost and performance will occur. FIG. 2 is a perspective view of a conventional magnetic storage device. Magnetic disk drive 10 includes a rotating magnetic storage medium 12 that, as mentioned above, can store tens of gigabytes of data in an area of only a few square inches. A head gimble assembly 14 (“HGA”) positions a recording head 16 with a transducer in close proximity to the surface of the magnetic storage medium 12 to enable data to be read from and written to the storage medium. An actuator assembly 18 rotates the HGA 14 during operation to position the transducer of the recording head 16 at the proper location over the rotating magnetic storage medium 12.
One of the most significant problems that has arisen in the effort to improve capacity and performance in magnetic storage devices is track following, or the ability to quickly and reliably position the transducer of the recording head 16 over the appropriate track on the magnetic storage medium 12. In conventional devices, the actuator assembly 18 includes a voice coil that uses a feedback loop based on servo tracks that are embedded between the data tracks on the magnetic storage medium 12. The track pitch (i.e., the spacing between adjacent tracks) of the storage medium 12 in conventional devices is as low as 0.2 microns. At such small track pitches, non-repeatable motions of the rotating magnetic storage medium 12, the HGA 14, and the other mechanical components of disk drive 10 make it increasingly difficult to reliably follow the data tracks on the magnetic storage medium. For example, in devices having an HGA 14 with a length of 1.5 inches to the recording head 16 and a track pitch of 0.2 microns, the angular position of the head gimble assembly needs to have resolution better than 33 millionths of an arc second in order to adequately follow the tracks on the magnetic storage medium 12. Efforts to achieve adequate track following have included the use of smaller disks for high speed drives, fluid motors for improved damping, and active rotational feedback sensors using negative feedback algorithms. However, the use of such techniques can lead to either the loss of capacity or are only temporary solutions to this problem, as track pitches continue to decrease.
A closely related problem is that of the settling time and performance, which relates to the ability to stabilize the recording head over a track. The settling time is dictated by the inertial loads and the exciting resonant frequencies associated with the act of accessing a selected track, the amount of damping in the HGA 14, and the servo bandwidth. These factors are generally limited by the resonant frequencies in the arm of the HGA 14. Thus, settling times have not significantly improved in the last several generations of drives in view of the fundamental limitations on the mechanics of drives that use a recording head 16 controlled by an HGA 14 and an actuator assembly 18, as shown in FIG. 2.
As both the track pitch and the size of sector regions on the magnetic media used to physically record bits of data have decreased, transducers in disk drives have been required to be positioned closer to the surface of the magnetic storage device. A representation of the distance between the transducer and the surface of the magnetic storage medium, referred to as the fly height 22, is shown in FIG. 3. Current fly heights are now as small as 50 Angstroms (Å) in high capacity disk drives. The fly height is dictated by the fundamental resolution requirements associated with the magnetic storage device, which is a function of the track pitch and the size of the regions on which bits of data are physically recorded. If the fly height becomes too large during operation, the transducer becomes unable to resolve bits encoded in the storage medium. On the other hand, if the transducer is brought into physical contact with the optical storage medium, which can be traveling at speeds on the order of 100 miles per hour, both the transducer and the storage device can be damaged.
The fly height has been controlled in conventional devices by improving the manufacturing tolerances, by designing a highly rigid and dampened HGA 14, and by the use of air bearings associated with the recording heads 16. An air bearing is a cushion or layer of air that develops between the surface of the magnetic storage medium and the adjacent surface of the transducer as the storage medium moves underneath the transducer.
As noted above, as the fly heights required in magnetic storage devices have decreased, the problem of transducer damage from excessive media contact has become more pronounced. Current giant magnetoresistance (“GMR”) and tunneling magnetoresistance (“TMR”) transducer heads are sensitive to being damaged if excessive contact with the storage medium is experienced. One related problem is that conventional transducer designs often lead to thermal pole tip protrusion, which occurs when the transducer is heated and the tip, or pole, of the transducer extends and protrudes beyond the plane of the transducer. Thermal pole tip protrusion can aggravate the contact of the transducer with the storage medium and can lead to increased or more rapid damage of the transducer.
These problems currently facing the magnetic storage device industry threaten to impede the ongoing progress in reliability, performance, and cost that has been achieved during recent years. Although many of these problems can be overcome to some degree using conventional head gimble assembly designs, it is unlikely that these problems can be successfully overcome while keeping costs for disk drive users down.
One approach that is currently being developed to lessen the effects of the challenges discussed above involves a technique called second stage actuation. Second state actuation systems use a dual actuation method for controlling the horizontal tracking position of the head over a servo mark positioned on the surface of the storage medium. A coarse actuator, similar to a HGA, positions the recording head to a global position, and a fine actuator with a single, horizontal degree of freedom at the head positions the head and transducer to a fine position. While this technique can be adequately practiced in connection with previous versions of magnetic storage media, the increased density on newer discs requires closer tolerances on the fly height, as discussed above. As the fly heights of newer storage systems continually decrease, second stage actuation technology becomes increasingly inadequate, particularly in light of the fact that transducer positioning is limited to adjustment in only the horizontal direction.
Additionally, it is known that previous methods have been attempted to measure fly height of a recording head above the surface of a magnetic storage medium. These methods include calculations involving capacitance, ratios of certain harmonic amplitudes, and vibrational aspects of piezo-electric devices mounted on the recording head. However, these methods have proven inadequate in precisely controlling and calibrating fly height and other possible movements of the recording head in newer magnetic storage devices.